(1) Field of the Invention
The present invention relates to thermal delayed-tack adhesive compositions which are non-adhesive at ordinary temperatures but which can be activated and tackified when heated, and which still further can maintain its tackiness for a long period of time even after being separated from a heat source, and it also relates to thermal delayed-tack adhesive sheets which are prepared by means of coating sheet materials with said thermal delayed-tack adhesive compositions and then drying the same.
(2) Description of the Prior Art
Thermal delayed-tack adhesive sheets are also called heat sensitive delayed adhesive sheets and are described in, for example, "Adhesion Handbook", 12th edition, Kobunshi Kanko-kai, 1980. Fundamentally, each of the thermal delayed-tack adhesive sheets has a coating layer which comprises a thermal delayed-tack adhesive composition containing (a) a polymeric resin such as, for example, polyvinyl acetate, polybutyl methacrylate, vinyl chloride-vinylidene chloride copolymer, a synthetic rubber, vinyl acetate-2-ethylhexyl acrylate copolymer, vinyl acetate-ethylene copolymer. vinylpyrrolidone-styrene copolymer, styrene-butadiene copolymer or vinylpyrrolidone-ethyl acrylate copolymer, (b) a plasticizer, which is in a solid state at ordinary temperatures, such as, for example, diphenyl phthalate, dihexyl phthalate, dicyclohexyl phthalate, dihydroabietyl phthalate, dimethyl isophthalate, sucrose benzoate, ethylene glycol dibenzoate, trimethylolethane tribenzoate, glyceride tribenzoate, pentaerythritol tetrabenzoate, sucrose octaacetate, tricyclohexyl citrate or N-cyclohexyl-p-toluenesulfonamide, and (c) a tackifier such as, for example, a rosin derivative (rosin, polymerized rosin, hydrogenated rosin, its ester with glycerin, pentaerythritol or the like, resin acid dimer, or the like), terpene resin, petroleum resin, phenolic resin or xylene resin. The above-mentioned polymer material is the basis of the tack strength and adhesive force, and the tackifier is the component for increasing the tackiness when the polymer material is activated by means of the heating process. The solid plasticizer is in the state of a solid at ordinary temperatures and does not provide any plasticity to the polymer material at such a temperature level, but when heated, the plasticizer is melted, so that the polymer material is expanded and dissolved in order to exhibit the tackiness. After being melted, the plasticizer crystallizes slowly, and therefore even after the thermal activation, the tacky state can remain upon the tack sheet for a long period of time. In addition, morphological examples of a known coating liquid for the manufacture of the thermal delayed tack sheets include a hot melt type liquid for example, Japanese Patent Unexamined Publication Nos. 55-16055 and 56-106984), an organic solvent type liquid for example, Japanese Patent Unexamined Publication No. 51-111246) and an aqueous type liquid (Souichi Muroi, "Polymer Latex Adhesive", Kobunshi Kanko-kai).
In the case that the composition prepared by means of blending the polymer resin, the tackifier and the solid plasticizer is of the hot melt type or organic solvent type (coating liquid), the solid plasticizer is melted by means of heating or by being disposed within an organic solvent. Therefore, the composition with which the sheet has just been coated is active and is in a tacky state. In order to render the composition nonactive, it is necessary to wait for the recrystallization of the solid plasticizer, and hence a process is required in which the coated sheets are allowed to stand for a very long period of time. In addition, when wound into a roll or superimposed upon each other, the coated sheets are liable to stick to each other. Therefore, the composition of such a type must be applied to specific basic sheets previously coated upon the other side thereof with a suitable releasing agent.
Moreover, conventional thermal delayed tack sheets tend to prematurely peel from articles upon which they have been stuck, particularly in the case wherein the articles are made from a plastic material such as, for example, polyethylene having low polarity. In consequence, it is fair to say that the conventional sheets have poor practical value.